The invention relates to a lubricator for underbalanced drilling.
There are two techniques that typically are used to drill a borehole in a formation: an overbalanced drilling technique and underbalanced drilling technique. In overbalanced drilling, fluid in an annulus of a well is used to exert a pressure that is greater than the formation pressure. Thus, the pressure that is exerted by the annulus fluid keeps formation fluids from exiting the well. A drawback to this technique is that mud particles typically are added to the annulus fluid to increase its weight (and thus, increase its downhole pressure), and these mud particles tend to clog up openings in the formation. Thus, the formation may be damaged by overbalanced drilling, and after drilling, cleanup of the well may be needed before production begins. The well may also need to be tested after overbalanced drilling to check for formation damage.
Unlike overbalanced drilling, underbalanced drilling typically does not damage the formation damage and typically maximizes reservoir inflow. In underbalanced drilling, heavy annulus fluid is not used to suppress the formation pressure. Instead, a blowout preventer, or snubbing unit, is used to seal off the drill string at the surface of the well. However, this arrangement may also present difficulties. For example, when drilling at shallow depths or retrieving the drill string, the upward force from the formation pressure may exceed the weight of the drill string and thus, may force the drill string out of the borehole. As a result, retrieving the drill string may consume a considerable amount of time and present a significant danger.
Thus, there is a continuing need for an arrangement to address one or more of the problems that are stated above.